Electrochromic devices are being increasingly used for automotive mirrors and have been suggested for many different applications. Recently, several publications suggest use of ionic liquids in electrolytes for EC devices, e.g. WO 03/003110, U.S. Pat. No. 6,365,301, Japanese application 08-329479 (publication number 10-168028). Published US patent application 20040021928 discloses EC devices using ionic liquids along with preferred characteristics of ionic liquids suitable for electrooptic devices, and the entire disclosure of that application is incorporated herein by reference.
Electrochromic (EC) automotive mirrors and other devices that can be fabricated from electrolytes comprising ionic liquids have several advantages such as:
Negligible vapor pressure even at high temperatures
Non-flammable
However, the preferred ionic liquids used in this invention have several other advantages, some of which are:
High electrochemical stability range
Insensitive to moisture absorption
Low UV susceptibility
Low corrosion
All of these characteristics lead to more durable EC devices.
FIG. 1 shows EC devices where an electrolyte 12 is sandwiched between two conductive, largely parallel substrates. The substrates 10, which are generally non-conductive, are pre-coated with a conductive material 11 on the inward facing surfaces. For windows, both the substrates and the coatings should be transparent (at least to the eye). Conductive transparent layers typically are indium tin oxide, fluorine doped tin oxide, etc. For mirrors at least one of these must be transparent. The other conductor may be a metal layer that also serves as a reflector, otherwise a reflector may be placed on one of the outwardly facing surface of the substrate. These are called single compartment devices as all electrochemical activity takes place within the electrolytic layer. Both electrochromic (EC) and electroluminscent (EL) devices may be made using such a construction. Such constructions are used for EC mirrors (e.g. automotive mirrors) for their self-erasing property, which means that the device spontaneously goes to a bleached state when the powering voltage is removed. The conductivity of the transparent conductors for automotive (and other transportation) rear-view mirrors is generally between 1 to 100 ohms/square. However, as disclosed in the U.S. patent application Ser. No. 10/741,903 filed on Dec. 19, 2003, electrolytes with higher ionic concentration may use higher resistance transparent conductors as compared to those devices which have lower ionic concentration. This application (application Ser. No. 10/741,903), which is incorporated herein by reference, also discloses that for automotive mirror applications, preferred electrolyte thickness is preferably lower than 250 microns.
The EC devices may contain other layers deposited on one of the electrodes. Schematics of such EC devices are shown in FIGS. 2 and 3. FIG. 2 shows the substrates 20 coated with conductive layers 21. An electrochemically active layer 23 is deposited on one of the conductive layers. Examples of such electrochemically active layers are tungsten oxide, Prussian blue, molybdenum oxide, vanadium oxide, polyaniline, polythiophene, and polypyrrole. Such layers may also include derivatives and mixtures of these materials. As an example, a commonly used derivative of polythiophene is poly-3,4-ethylenedioxythiophene. That material is useful in EC mirrors that are intended to have self-erasing property. FIG. 2 also shows another kind of EC device where the layer 23 changes its electrochromic properties from reflection to transmission. For example, Richardson, T. J. et al (Richardson, T. J., et al, “Lithium based EC Mirrors”, Proceedings of the Electrochemical Society, (2003)) describe the layer composition as metal hydrides and their alloys, mixtures of magnesium and transition metals, and other metals such as copper, antimony, bismuth and silver. For example antimony doped with copper or silver changes reversibly from being reflective to being transmissive when reduced with lithium in the electrochemical cell.
FIG. 3 shows a device where each of the substrates 30 is coated with transparent conductor 31. One transparent conductor is coated with a material 33 (as described in Example 2, layer 23), such as tungsten oxide. The tungsten oxide is further coated with an ion-selective transportation layer 34 which primarily allows e.g., lithium to go through but blocks or retards the motion of the larger ions present in the electrolyte 32 (see U.S. Pat. No. 6,178,034). The electrolyte composition is usually the same as in Example 2. This limits the back reaction and increases the memory of the EC device. This construction is useful for large area windows to conserve power and allow uniform coloration. These may be used for visors, contrast enhancement filters for large displays, automotive and architectural windows.
FIG. 4 shows substrates 40, each coated with a conductive transparent layer 41. Each conductive transparent layer 41 is further coated with one additional layer (e.g. layer 43 or 45). One of these layers, e.g., layer 43 has to be electrochromic; the other layer (counterelectrode or the complimentary layer, CE) may be electrochromic or only store the ions reversibly. If the EC layer comprises tungsten oxide and molybdenum oxide, the CE can comprise polyaniline, nickel oxide, iridium oxide and vanadium oxide for electrochromic intercalatable layers. Some examples of non-electrochromic electrodes are cerium-titanium and vanadium-titanium oxide. Typically these EC devices have good memory and are useful for large area devices.
FIGS. 1 and 2 generally show the schematic structures of EC devices which are being used for commercial mirrors today. All presently fabricated commercial automotive EC mirrors have at least one redox dye (e.g. FIG. 2), and most have at least two redox dyes (e.g., FIG. 1) in the electrolytic medium. In FIG. 1, the electrolytic medium is in contact with the two opposing electronically conductive surfaces of the cell and in FIG. 2 there is a complimentary electrochemically active layer inserted between one of the electronically conductive surface and the electrolyte. In both cases at least one of the dyes or the electrochemically active layer is electrochromic. Electrochromic material is one which reversibly colors when it is either oxidized or reduced by an electric stimulus.
Almost all of the commercial mirror devices are made by backfilling an empty cavity with a liquid electrolyte or a liquid material which later reacts in-situ to form a solid electrolyte. Backfilling is conducted using a vacuum apparatus. Since the conventional electrolytic solvents have high vapor pressures, some of these evaporate during the back-filling process and thus contaminate the equipment. The equipment has to be cleaned periodically resulting in downtime. Since the ionic liquids have negligible vapor pressure this contamination is reduced for electrolytes comprising of ionic liquids resulting in a higher efficiency manufacturing process. The ionic liquids maintain negligible vapor pressure even at elevated temperatures, thus filling at elevated temperatures (preferably between 50 to 120° C.) can be carried out to lower the electrolyte viscosity and increase the back-fill rate. For backfilling at elevated temperatures the electrolyte and/or the cavity to be filled are pre-heated or heated during the operation.
The disclosure below relates to electrochromic (EC) device configurations and techniques that are particularly useful in forming automotive rearview mirrors, and many of which configurations and techniques are usable for any other application of electrochromic (EC) and other electro optic devices including transmissive type.
One objective of the present invention is to disclose novel reflective electrode compositions for EC mirrors.
Another objective is to disclose novel transparent conductors for EC mirrors.
Yet another objective is to disclose processes to deposit reflective layers and transparent conductors for electrochromic mirrors.
Still another objective is to disclose busbar patterns for electrochromic mirrors.
Yet another objective is to disclose sealant material compositions for electrochromic assemblies.
Another objective is to disclose integration of displays and indicators on EC mirrors
Still another objective of this invention is to disclose compositions for solid electrolytes for use in electrooptic devices.
Yet another objective is to disclose electronic control circuits for electrochromic mirrors.